Large volcanic eruptions with the volcanic explosivity
index (VEI) ≥ 3 are widely known to be the strongest source of
long-lived aerosol in the upper troposphere and lower stratosphere (UTLS).
However, the latest studies have revealed that massive forest (bush) fires
represent another strong source of short-term (but intense) aerosol
perturbations in the UTLS if combustion products from the fires reach these
altitudes via convective ascent within pyrocumulonimbus clouds (pyroCbs).
PyroCbs, generated by boreal wildfires in North America and northeastern Asia
and injecting smoke plumes into the UTLS, have been intensively studied
using both ground- and space-based instruments since the beginning of the
21st century. In this paper, we focus on aerosol layers observed in the UTLS over
Tomsk (56.48∘ N, 85.05∘ E, Western Siberia, Russia) that
could be smoke plumes from such pyroCb events occurring in the
2000–2017 period. Using the HYSPLIT trajectory analysis, we have reliably assigned
nine aerosol layers to 8 out of more than 100 documented pyroCb events,
the aftereffects of which could potentially be detected in the UTLS over
Tomsk. All the eight pyroCb events occurred in the USA and Canada: one event
per year occurred in 2000, 2002, 2003, 2013, 2015, and 2016, whereas two events
occurred in 2017. No plumes from pyroCbs originating in the boreal zone of
Siberia and the Far East (to the east of Tomsk) were observed in the UTLS
over Tomsk between 2000 and 2017. We conclude that the time durations for
pyroCb plumes to be detected in the UTLS using ground-based lidars are less
than about a month, i.e., plumes from pyroCbs generated by wildfires to the
east of Tomsk can significantly diffuse before reaching the Tomsk lidar
station by the westerly zonal transport of air masses. A comparative
analysis of the contributions from pyroCb events and volcanic eruptions with
VEI ≥ 3 to aerosol loading of the UTLS over Tomsk showed the following.
Plumes from two or more pyroCbs that have occurred in North America in a
single year are able to markedly increase the aerosol loading compared to
the previous year. The annual average value of the integrated aerosol
backscatter coefficient Bπ,532a increased by 14.8 %
in 2017 compared to that in 2016 due to multiple pyroCbs occurring in British
Columbia (Canada) in August 2017. The aftereffects of pyroCb events are
comparable to those of volcanic eruptions with VEI ≤ 3, but even
multiple pyroCbs can hardly compete with volcanic eruptions with VEI = 4.

There are many sources of aerosol in the troposphere: bio- and fossil-fuel
burning, forest and bush fires, power generation and industrial processes,
engines, volcanic eruptions, etc., and conversely, only a few such sources
exist in the stratosphere. Aircraft emissions (combustion products of
carbon-containing fuels) (Blake and Kato, 1995; Hendricks et al., 2004;
Koehler et al., 2009; Wilkerson et al., 2010; Balkanski et al., 2010) and
troposphere-to-stratosphere transport of air (Kremser et al., 2016) are
responsible for background aerosol loading in the lower stratosphere (LS).
Large volcanic eruptions with the volcanic explosivity index (VEI) ≥ 3
represent the principal source of strong and long-term stratospheric aerosol
perturbations (Robock, 2000; Robock and Oppenheimer, 2003; Kremser et al.,
2016), which is confirmed by both space-borne and ground-based long-term
lidar measurements (Vernier et al., 2011; Trickl et al., 2013; Mills et al.,
2016; Sakai et al., 2016; Khaykin et al., 2017; Zuev at al., 2017; Friberg et
al., 2018). Volcanic plumes persist in the stratosphere for several months
to several years, depending on the eruption latitude, VEI, and maximum plume
altitude (MPA) after the eruptions (Hofmann et al., 2009). However, studies
over the last two decades have revealed that, in addition to volcanic
eruptions, there exists another source able to cause short-term, but
locally intense, aerosol perturbations in the LS. This source is massive
forest (or bush) fires if combustion products from the fires reach
stratospheric altitudes.

Massive forest fires (wildfires), the plumes of which can ascend to the LS,
and their aftereffects have been intensively studied since the beginning of
the 21st century (Fromm et al., 2000, 2005, 2006, 2008a, b, 2010; Fromm and
Servranckx, 2003; Jost et al., 2004; Livesey et al., 2004; Damoah et al.,
2006; Cammas et al., 2009; Gonzi and Palmer, 2010; Guan et al., 2010;
Siddaway and Petelina, 2011; Dahlkötter et al., 2014; Paugam et al.,
2016). Smoke plumes of the overwhelming majority of forest fires are located
within the planetary boundary layer (Val Martin et al., 2010; Nikonovas et
al., 2017; Rémy et al., 2017), and a small number of them (<5 %–10 %)
can enter the free troposphere (Sofiev et al., 2013; Peterson et
al., 2014). Only in exceptional cases are aerosol plumes from the fires able
to reach stratospheric altitudes via convective ascent within
pyrocumulonimbus clouds (pyroCb;
http://glossary.ametsoc.org/wiki/Pyrocumulonimbus, last access: 13 March 2019). PyroCbs, injecting
aerosol directly into the LS, originate mainly from boreal wildfires in
North America (particularly in the Canadian boreal zone) and northeastern Asia
(Siberia and the Far East) (Fromm et al., 2010; Guan et al., 2010), and bush
fires in Australia (Fromm et al., 2006; Siddaway and Petelina, 2011). In
particular years, pyroCb events can occur too frequently to be considered as
an occasional source of aerosol in the LS. For example, Fromm et al. (2010)
identified 17 such pyroCbs in the US and Canada during the summer
of 2002, a part of which reached the LS.

PyroCb stratospheric plumes can spread throughout the hemisphere and are
detected by both ground- and space-based lidars for 2 to 4 months after
their occurrence (Fromm et al., 2000, 2008b, 2010). Owing to their potential
impact on the climate, a lot of attention is currently paid to monitoring
pyroCbs via, for example, the Geostationary Operational Environmental Satellite (GOES)
system (https://www.nasa.gov/content/goes, last access: 13 March 2019). The data on pyroCb events
occurring throughout the world are accumulated on the web page of the
Cooperative Institute for Meteorological Satellite Studies (CIMSS):
http://pyrocb.ssec.wisc.edu/ (last access: 13 March 2019)
and their archives have been available since May 2013.

Ground-based lidar observations of stratospheric aerosol perturbations have
been almost continuously performed in Tomsk (56.48∘ N,
85.05∘ E, Western Siberia, Russia) for more than 30 years (Zuev et
al., 1998, 2001, 2017). In the papers, we mainly discussed and focused on
aerosol perturbations in the stratosphere over Tomsk after major volcanic
eruptions (with VEI ≥ 3), the plumes of which were able to directly
enter the stratosphere. To consider the effect of only volcanic eruptions on
stratospheric aerosol loading and definitely exclude from consideration any
aerosol perturbations in the upper troposphere (UT) (such as cirrus clouds)
and tropopause region (TR), we analyzed the results of lidar measurements at
altitudes higher than 13–15 km. It is clear that this altitude limitation
could lead to the loss of information on aerosol events like pyroCb plumes
in the upper troposphere and lower stratosphere (UTLS) over Tomsk. The possibility to observe stratospheric smoke
plumes in Tomsk from massive forest fires occurring in North America was
noted in Zuev et al. (2017). In this paper, we analyze aerosol perturbations
in the 11–30 km altitude region over Tomsk that could be caused by massive
wildfires in North America and northeastern Asia from 2000 to 2017.

The lidar measurements we consider were made using the aerosol channel of
the Siberian Lidar Station (SLS) located in Tomsk. The transmitter of the
channel represents a Nd:YAG laser (LS-2132T-LBO model, LOTIS TII Co., the
Republic of Belarus) that operates at a wavelength of 532 nm with 100 mJ
pulse energy and at a pulse repetition rate of 20 Hz. The channel receiver
is a Newtonian telescope with a mirror diameter of 0.3 m and a focal length
of 1 m. The backscattered signals are registered by a photomultiplier tube
R7206-01 (Hamamatsu Photonics, Japan) operating in the photon counting mode
with a vertical resolution of 100 m. Owing to the rearrangement and
improvement of the SLS, there were two shutdown periods of the aerosol
channel from July 1997 to May 1999 and from February to September 2014. A
detailed description of the SLS aerosol channel technical parameters is
given in Zuev (2000) and Burlakov et al. (2010).

The scattering ratio R532(H) is used to describe the aerosol vertical
distribution in the UTLS:

(1)R532(H)=βπ,532m(H)+βπ,532a(H)βπ,532m(H)=1+βπ,532a(H)βπ,532m(H).

Here βπ,532m(H) and βπ,532a(H) are
the altitude-dependent molecular (Rayleigh) and
aerosol (Mie) backscatter coefficients, respectively; π denotes the
angle of the backscatter lidar signal propagation (i.e., π radian).
Stratospheric altitudes of ∼30–35 km over Tomsk are mostly
aerosol-free and, therefore, we use an altitude H0=30 km for
calibrating the detected lidar signals by normalizing them to the molecular
backscatter signal from H0 (Zuev et al., 2017).

The integrated aerosol backscatter coefficient Bπ,532a
is used to discover the temporal dynamics of aerosol loading in the UTLS
over Tomsk in the 2001–2017 period:

(2)Bπ,532a=∫H1H2βπ,532a(H)dH,

where the lower limit H1=11 km can fall within the UT, TR, or LS due
to the variability of the local tropopause altitude, and the upper limit is
the calibration altitude H2=H0=30 km. The use of the fixed
11 km altitude is a compulsory measure because there is a problem in
determining the tropopause altitude over the lidar site due to the absence
of a meteorological station launching radiosondes in Tomsk. Nevertheless,
the 11 km lower limit does not allow missing pyroCb plumes from North
America and excludes the tropospheric aerosol sources with the exception of
cirrus clouds. Moreover, the fixed 11–30 km altitude region allows us
(regardless of the real tropopause altitude) to make a comparative analysis
of aerosol loading over Tomsk due to both volcanic eruptions and pyroCb
events from 2000 to 2017.

To estimate the tropopause altitude over the lidar site, we use vertical
temperature profiles from three neighboring meteorological stations launching
radiosondes twice a day (at 00:00 and 12:00 UTC). These stations are located
in Kolpashevo (58.31∘ N, 82.95∘ E), Emeljanovo
(56.18∘ N, 92.61∘ E), and Novosibirsk (54.96∘ N,
82.95∘ E) (Fig. 1), the radiosonde data of which can be found
on the web page http://weather.uwyo.edu/upperair/sounding.html?region=np
(last access: 13 March 2019) of the University of Wyoming (Kolpashevo, Emeljanovo, and Novosibirsk
station numbers are 29231, 29572, and 29634, respectively). The lower
boundary of the tropopause is determined by the temperature lapse rate of
2 K km−1 according to the criterion provided by the World Meteorological
Organization (WMO, 1957). All temperature profiles and estimated tropopause
altitudes that we used in our study are also given in the Supplement. When
analyzing perturbed scattering ratio R532(H) profiles, cirrus clouds are
excluded from consideration based on the criteria presented in Appendix A.

Figure 1Three neighboring meteorological stations (with the distances between
them and Tomsk) launching radiosondes twice a day. The stations are numbered
for convenience.

We took data on pyroCb events from scientific papers if the events were
documented from 2000 to 2012, and at http://pyrocb.ssec.wisc.edu/ (last access: 13 March 2019) for
pyroCbs that occurred after May 2013. The required data on volcanic eruptions for
the 2004–2017 period were taken from the Smithsonian Institution Global
Volcanism Program (GVP; http://volcano.si.edu/, last access: 13 March 2019; Section: Reports;
Subsections: Smithsonian/USGS Weekly Volcanic Activity Report and Bulletin
of the Global Volcanism Network). To assign aerosol layers detected in the
UTLS over Tomsk to their sources (pyroCbs or volcanic eruptions), we analyze
air mass backward trajectories calculated with the NOAA's PC Windows-based
HYSPLIT trajectory model (February 2018 Release; Stein et al., 2015;
http://ready.arl.noaa.gov/HYSPLIT.php, last access: 13 March 2019), the HYSPLIT-compatible NOAA
meteorological data from the NCEP/NCAR Reanalysis (2000 to 2003), and the Global
Data Assimilation System (GDAS) one-degree (July 2013 and May to August 2017)
and half-degree (September 2013 to May 2016) archives.

All altitudes in this study are given above mean sea level (m.s.l.),
whereas all dates and times are given in UTC.

Figure 2(a) Detection of the Jasper Fire pyroCb plume in the UTLS
over Tomsk. The numbers 2 and 3 indicate the tropopause altitudes estimated in
Emeljanovo and Novosibirsk, respectively, at 19:30 UTC on 4 September 2000.
(b) Air mass backward ensemble trajectories started from altitudes of
12.0–12.15 km over Tomsk at the same time.

There were no eruptions of either tropical or northern volcanoes to be
recorded at the SLS in Tomsk from the middle of 2000 to the end of 2004 and
in the 2012–2016 period (Zuev et al., 2017), with the exception of the
13 February 2014 Kelut eruption, the plume of which, however, could not be
detected at the SLS due to the 2014 shutdown period (see Sect. 2). When
analyzing aerosol layers observed over Tomsk and pyroCb events documented in
the Northern Hemisphere over the period 2000–2017, we have discovered more
than 100 pyroCbs (with known and unknown MPAs), the plumes of which could
potentially be detected in the UTLS over Tomsk. However, only a few of the
detected layers have been reliably attributed to the selected pyroCb events
using the HYSPLIT trajectory analysis. To illustrate the correlation between
the pyroCbs and corresponding layers over Tomsk, we present only the most
successful examples of the HYSPLIT trajectories that passed over or close to
the places of origin of the pyroCbs (or near the known pyroCb plume
locations when the exact coordinates and time of the pyroCb events are
unknown). We have also analyzed available CALIPSO data to corroborate the
Tomsk observations for the 2013–2017 period. The CALIPSO data are given in the Supplement.

3.1 Detection of pyroCb smoke plumes in the UTLS

The first aerosol layer we consider was observed in the TR over Tomsk at
altitudes between 11.4 and 12.5 km with the maximum R(H)=2.42 at H=12.1 km
on 4 September 2000 (Fig. 2a). The HYSPLIT trajectory analysis showed
that this layer was highly likely to be a smoke plume initially detected
in the UTLS over Iowa (∼42∘ N, ∼92∘ W; USA)
by the Total Ozone Monitoring Spectrometer (TOMS) on 27 August (Fromm et
al., 2010). The plume originated from a pyroCb that occurred due to the
massive “Jasper Fire” in the Black Hills National Forest (South Dakota,
USA). Figure 2b shows, as an example, three air mass backward trajectories
started from altitudes of 12.0–12.15 km over Tomsk at 19:30 on 4 September
and passed close to the Jasper Fire pyroCb plume location at altitudes
Htraj.back. of 9.3–10.2 km on 27 August. According
to radiosonde data from two close stations located in Green Bay
(44.48∘ N, 88.13∘ W; USA) and Davenport
(41.61∘ N, 90.58∘ W; USA), the tropopause was at
15.0–15.9 km on that day. Based upon the end points of the trajectories
(with Htraj.back.≤10.2 km) and tropopause
altitude, the Jasper Fire smoke came to the Tomsk TR from the UT over the
place of the pyroCb plume observation in Iowa.

According to Fromm et al. (2010), a pyroCb generated by the “Mustang Fire”
was registered with HMPA=13 km on the border of Utah and Wyoming
(41.0∘ N, 109.3∘ W; USA) on 1 July 2002. On 15 July, 2 weeks after
the event, an aerosol layer was observed in the LS over Tomsk at
altitudes of ∼11.7–13.5 km with the maximum R(H)=1.41 at H=12.4 km
(Fig. 3a). Figure 3b presents the HYSPLIT air mass backward ensemble
trajectories started from altitudes of ∼12.1 km over Tomsk at 17:00 on
15 July and passed near the place of origin of the Mustang Fire pyroCb at
altitudes Htraj.back.≈11.9–12.4 km on
1 July. The analysis of radiosonde data from the meteorological station
located in Riverton (43.06∘ N, 108.48∘ W; USA) revealed
that the tropopause altitude was around 13.0 km on that day and, therefore,
we can assume that the pyroCb MPA did not exceed the local tropopause. Air
masses containing the Mustang Fire pyroCb plume came to the Tomsk LS from
the UT over a place close to the pyroCb origin.

Figure 3(a) Detection of the Mustang Fire pyroCb plume in the LS over
Tomsk. The numbers 2 and 3 indicate the tropopause altitudes estimated in
Emeljanovo and Novosibirsk, respectively, at 17:00 UTC on 15 July 2002.
(b) Air mass backward ensemble trajectories started from altitudes of
∼12.1 km over Tomsk at the same time.

Figure 4(a) Detection of the Conibear Lake Fire pyroCb plume in the
UT–TR over Tomsk. The numbers 1–3 indicate the tropopause altitudes estimated
in Kolpashevo, Emeljanovo, and Novosibirsk, respectively, at 17:00 UTC on
29 August 2003. (b) Air mass backward ensemble trajectories started
from altitude of ∼11.7 km over Tomsk at the same time.

Another aerosol layer potentially associated with a pyroCb event was
observed in the UT–TR over Tomsk between 10 and 12 km with the maximum
R(H)=1.87 at H=11.1 km on 29 August 2003 (Fig. 4a). On 18 August, 11 days
earlier, a pyroCb plume was registered over Hudson Bay
(61∘ N, 89∘ W; Canada) by the TOMS. The pyroCb was
previously generated by the “Conibear Lake Fire” in the Wood Buffalo
National Park (Alberta–Northwest Territories, Canada) (Fromm et al., 2010).
As seen in Fig. 4b, the HYSPLIT air mass backward trajectories, started from
altitudes of ∼11.7 km over Tomsk at 17:00 on 29 August, passed over
the pyroCb plume location at altitudes Htraj.back.≈11.7–11.9 km
on 18 August. Radiosonde data from the Churchill
station (58.73∘ N, 94.08∘ W; Canada) to the west of the
plume registration showed the tropopause altitude of ∼11.4 km.
According to the Inukjuak station (58.45∘ N, 78.11∘ W;
Canada) to the east of the plume registration, the tropopause was at
∼11.9 km on that day. Thus, we can conclude that the Conibear Lake Fire
smoke came to the Tomsk UT–TR from the UT–TR over Hudson Bay.

Figure 5(a) Detection of the Eastmain pyroCb plume in the UT–TR over
Tomsk. The numbers 1–3 indicate the tropopause altitudes estimated in Kolpashevo,
Emeljanovo, and Novosibirsk, respectively, at 17:30 UTC on 14 July 2013.
(b) Air mass backward ensemble trajectories started from altitude of
10.6–10.7 km over Tomsk at the same time.

The next aerosol layer reliably attributed to a pyroCb event was registered
at the SLS in Tomsk 10 years later, in July 2013. Namely, the layer with the
maximum R(H)=1.27 at H=11.7 km was observed in the UT–TR over Tomsk on
14 July 2013 (Fig. 5a). The HYSPLIT trajectory analysis showed that the
layer could represent a smoke plume from a pyroCb generated by large fires
that were burning in the Eastmain region of Quebec (∼52∘ N,
∼78∘ W; Canada) in June–July 2013. The Eastmain pyroCb was
discovered using the 1 km resolution GOES-13 0.63 µm visible channel
after 21:55 on 4 July (http://pyrocb.ssec.wisc.edu/archives/136, last access: 13 March 2019). Figure 5b shows
three example HYSPLIT air mass backward trajectories started from altitudes
of 10.6–10.7 km over Tomsk at 17:30 on 14 July and passed near the place of
origin of the Eastmain pyroCb at altitudes Htraj.back.
of 5.5–7.2 km on 4 July. The analysis of radiosonde data from the station
located in Moosonee (51.26∘ N, 80.65∘ W; Canada)
revealed that the tropopause altitude was varying from 10.9 to 11.8 km
during the afternoon of 4 July, whereas the data from the La Grande Iv
station (53.75∘ N, 73.66∘ W; Canada) showed that the
local tropopause was around 11.0 km at 00:00 on 5 July. Based upon the
behavior of the HYSPLIT trajectories (Fig. 5b), the pyroCb plume was
spreading in the middle and upper troposphere before reaching the UT–TR over
Tomsk on 14 July. Note that the aftereffects of the summer 2013 fire season
in North America were also widely observed in the troposphere over central
Europe (Trickl et al., 2015; Markowicz et al., 2016).

Only one aerosol layer associated with pyroCb events was detected in the
TR–LS over Tomsk in 2015. More precisely, the layer was observed between
10.1 and 12.0 km, with the maximum R(H)=1.62 at H=11.0 km on 16 July
(Fig. 6a). On 11 July, 5 days before, two pyroCbs were registered in
British Columbia using the GOES-15 0.63 µm visible, 3.9 µm IR, and
10.7 µm IR channels (http://pyrocb.ssec.wisc.edu/archives/985, last access: 13 March 2019). The former
pyroCb was observed at 56.4∘ N, 123.9∘ W with
HMPA=10.5 km around 00:30, whereas the latter one was detected at
52.2∘ N, 124∘ W with HMPA=10 km 4.5 h
later, at ∼05:00. As seen in Fig. 6b, three HYSPLIT backward
trajectories that started from altitudes of 10.3–10.5 km over Tomsk at 18:00
on 16 July passed near and over the place of the former pyroCb origin
at altitudes Htraj.back.≈11.0–11.5 km on
11 July. According to the station located in Prince George (53.90∘ N,
122.80∘ W; Canada), the lower boundary of the tropopause was at
∼12.2 km at 00:00 on that day. Therefore, the pyroCb plume
with HMPA=10.5 km did not exceed the local tropopause and came to the
TR–LS over Tomsk from the UT over the place of the pyroCb origin in British
Columbia. We could not connect the aerosol layer under consideration with
the latter pyroCb event.

Figure 6(a) Detection of a pyroCb plume from British Columbia in the
TR–LS over Tomsk. The numbers 1–3 indicate the tropopause altitudes estimated
in Kolpashevo, Emeljanovo, and Novosibirsk, respectively, at 18:00 UTC on
16 July 2015. (b) Air mass backward ensemble trajectories started
from altitude of 10.3–10.5 km over Tomsk at the same time.

Figure 7(a) Detection of a pyroCb plume from British Columbia in the
TR–LS over Tomsk. The numbers 1–3 indicate the tropopause altitudes estimated
in Kolpashevo, Emeljanovo, and Novosibirsk, respectively, at 17:30 UTC on
27 May 2016. (b) Air mass backward ensemble trajectories started from
altitude of ∼11.7 km over Tomsk at the same time.

Another marked pyroCb formed in British Columbia was observed at
∼56∘ N, ∼122∘ W by the GOES-15 visible and
IR channels at 22:00 on 16 May 2016
(http://pyrocb.ssec.wisc.edu/archives/1622, last access: 13 March 2019). On
27 May, 11 days after the event, a thin aerosol layer with a thickness of ∼0.8 km and the
maximum R(H)=2.48 at H=11.3 km was detected in the TR–LS over Tomsk
(Fig. 7a). Figure 7b presents the HYSPLIT backward ensemble trajectories
started from altitudes of ∼11.7 km over Tomsk at 17:30 on 27 May and
then passed close to the place of the pyroCb origin at altitudes
Htraj.back.≈10.6–10.8 km on 16 May.
Radiosonde data from the Fort Nelson station (58.83∘ N,
122.60∘ W; Canada) showed the tropopause altitude of around
11.1 km on that day. Hence, air masses containing the pyroCb plume came to the
Tomsk TR–LS from the UT over a place close to the pyroCb origin. The smoke
from the pyroCb was also observed in the UTLS over the UK with Raman lidars
between 23 and 31 May 2016 (Vaughan et al., 2018).

Table 1List of biomass burning plumes with HMPA≥12 km that
were documented in the Northern Hemisphere and could potentially be detected
in the UTLS over Tomsk in the 2004–2011 period. MPA: maximum plume altitude.

In August 2017, massive forest fires in British Columbia generated several
strong pyroCbs, the plumes of which reached stratospheric altitudes. The
aftereffects of these Canadian wildfires and pyroCb events were widely
observed in the UTLS over Europe in August and September 2017 and have
already been intensively studied by different research groups (Ansmann et
al., 2018; Haarig et al., 2018; Khaykin et al., 2018; Hu et al., 2019;
Peterson et al., 2018). We could attribute three aerosol layers observed in
the LS over Tomsk at the end of August to two out of five pyroCbs detected
by the GOES-15 instruments at 51.8∘ N, 123.2∘ W and
53.1∘ N, 121.0∘ W at around 03:30 and 05:30 on 12 August,
respectively (http://pyrocb.ssec.wisc.edu/archives/2135, last access: 13 March 2019). The first aerosol
layer was observed between 15.0 and 15.8 km with the maximum R(H)=5.8 at
H=15.4 km 2 weeks after the event, on 26 August (Fig. 8a). On 29 August, 3 days
later, the second layer was detected with the maximum
R(H)=1.37 at H=14.5 km (Fig. 8b). Finally, the third layer was observed
between 14.3 and 16.2 km with the maximum R(H)=3.1 at H=15.7 km on
31 August (Fig. 8c). The initial conditions (times and altitudes over Tomsk)
for each HYSPLIT trajectory can be found in Fig. 9. All the HYSPLIT backward
trajectories that started over Tomsk on 26, 29, and 31 August passed close
to the places of origin of both pyroCbs at altitudes
Htraj.back. of 13.7–13.9, 13.5–13.7, and
14.4–15.1 km, respectively, on 12 August. According to the stations located
in Prince George (53.90∘ N, 122.80∘ W; Canada) and Port
Hardy (50.68∘ N, 127.36∘ W; Canada), the tropopause
altitude was not higher than 12.3 km on that day. Therefore, based upon the
behavior of the HYSPLIT trajectories (Fig. 9), smoke plumes from both
pyroCbs were spreading in the LS before reaching Tomsk at the end of August.
This conclusion is also confirmed by the CALIPSO data (see the Supplement).
We cannot exclude that the layers observed over Tomsk on 26, 29, and
31 August could contain aerosol from the other three pyroCbs detected by the
NOAA-18 instruments on the evening of 12 August.

Figure 9Air mass backward ensemble trajectories started from altitude of
(a)∼15.75 km over Tomsk at 17:00 UTC on 26 August 2017,
(b)∼14.55 km at 16:00 UTC on 29 August 2017, and
(c)∼14.9 km at 15:00 UTC on 31 August 2017.

3.2 PyroCb events in 2004–2012

Several biomass burning plumes with HMPA≥12 km, which resulted
from pyroCbs and could potentially be detected over Tomsk, were documented
in the UTLS of the Northern Hemisphere between 2004 and 2011 (Table 1).
However, no aerosol layers associated with these plumes were observed at the
SLS during the period. This was due to unfavorable weather conditions (rain,
snow, fog, clouds) in Tomsk, or pyroCb plumes could have diffused or passed
by the SLS and, therefore, might not be detected. Note also that
12 explosive eruptions with VEI = 3–4 of both tropical and northern
extratropical volcanoes, the aftereffects of which were reliably registered
in the stratosphere over Tomsk, occurred in the 2004–2011 period (Zuev at
al., 2017). We do not exclude that pyroCb plumes could hardly be discerned
against the background of the volcanic plumes in the UTLS over Tomsk in this
period. There were no significant events (volcanic eruptions and pyroCbs) to
be recorded at the SLS in 2012.

4 Time series of the integrated aerosol backscatter coefficient (2001–2017)

To estimate the contribution of the pyroCb events discussed above to aerosol
loading of the UTLS over Tomsk, we have analyzed the 2001–2017 time series
of the annual average Bπ,532a values (see Sect. 2). The
upper part of Fig. 10 presents both the 10-day and annual average Bπ,532a
values obtained from the SLS observations. Due to
weather conditions in Tomsk, the observations are often irregular in time
and periods without lidar measurements can last up to several months. To
obtain a homogeneous time series of Bπ,532a values for
the time intervals when measurement data are available, all the data for
every 10-day period are averaged. The average values for the periods from
days 1 to 10, 11 to 20, and 21 to 30 (31) of a month are assigned to the 5th,
15th, and 25th days of the month, respectively. The same
data processing method was used in Zuev et al. (1998, 2017). The annual
average Bπ,532a values are assigned to 1 July of the
corresponding year. PyroCb events and volcanic eruptions (Tables 2 and 3),
the plumes of which were observed in the UTLS over Tomsk between 2000 and 2017,
are indicated by red and black vertical bars, respectively, in the lower part of Fig. 10.

Figure 10The 2001–2017 time series of the integrated aerosol backscatter
coefficient Bπ,532a obtained from lidar observations at
λ=532 nm in the 11–30 km altitude range. Open dark-green circles
denote the 10-day average Bπ,532a values; solid red circles
show the annual average Bπ,532a values assigned to 1 July of
each year. Red and black vertical bars in the lower part of the figure indicate,
respectively, pyroCbs and volcanic eruptions (see also Tables 2 and 3), the
plumes of which were observed in the UTLS over Tomsk between 2000 and 2017. The
black sloping lines show the trends in the annual average Bπ,532a
values for the 2001–2004, 2004–2011, and 2011–2017 periods.

The time series can be divided into three periods with different trends in
the annual average Bπ,532a values: 2001–2004,
2004–2011, and 2011–2017 (Fig. 10). The first 2001–2004 period (a) is
marked by a negative trend in the values caused by the absence of volcanic
eruptions with VEI ≥ 3 in the Northern Hemisphere. Despite the fact
that three pyroCb plumes were detected from 2000 to 2003, the minimum
(background) annual average Bπ,532a=3.07×10-4 sr−1
was reached in 2004. Note that when integrating the aerosol
backscatter coefficient βπ,532a(H) over the
15–30 km altitude range, the minimum annual average
Bπ,532a=1.29×10-4 sr−1 is also reached in 2004 (Zuev et al., 2017).
During the second 2004–2011 period (b), the aftereffects of 12 volcanic
eruptions (Table 3) measurably perturbed the UTLS over Tomsk and, therefore,
were registered at the SLS. This volcanic activity led to a positive trend
in the annual average Bπ,532a values. The last
2011–2017 period (c) is characterized by comparatively low activity of both
tropical and northern volcanoes. Only the 2014 Mt. Kelut volcanic eruption
could slightly perturb the UTLS over Tomsk in a given period of time
(Table 3). Thus, a negative trend in the annual average
Bπ,532a values was observed in spite of five pyroCbs that
injected smoke into the UTLS in 2013 and 2015–2017 (Table 2).

The trends in Fig. 10 show that for the Tomsk region the aftereffects of
tropical and northern volcanic eruptions with VEI ≥ 3 are stronger and
longer-lasting than those of pyroCb events that occurred mainly due to
wildfires in North America. Indeed, volumes and lifetimes of primary
(volcanic ejecta) and secondary (sulfuric acid) aerosols in the UTLS from
explosive volcanic eruptions are known to be higher (Hofmann et al., 2009)
compared to those of aerosols from pyroCb plumes (Fromm et al., 2010).
Hence, volcanic eruptions for period (b) naturally led to an increase in
aerosol loading of the UTLS over Tomsk and, therefore, to a positive trend
in the annual average Bπ,532a values. PyroCbs generated
by wildfires from 2004 to 2011 (including documented ones listed in Table 1)
also had to perturb the UTLS over Tomsk. But the use of our
single-wavelength lidar with no depolarization information makes it
impossible to unambiguously discern the pyroCb plumes against the background
of more powerful volcanic plumes for the same period. Nevertheless, a
comparison of the annual average Bπ,532a values in
periods (a) and (c) of volcanic quiescence with those in period (b) of
volcanic activity shows that the positive trend in the period 2004–2011
should have been mostly caused by volcanic eruptions. The same conclusion
was reached by Zuev et al. (2017) when integrating βπ,532a(H)
over the 15–30 km altitude range.

The presence of pyroCb plumes in the UTLS over Tomsk in periods (a) and (c)
did not allow the annual average Bπ,532a values to
decrease to the background level (as it was in 2004 in the absence of both
volcanic and pyroCb plumes). On the other hand, plumes from two or more
pyroCbs that have occurred in North America in a single year are able to
markedly increase aerosol loading of the UTLS over Tomsk compared to the
previous year (Fig. 10). For example, the annual average Bπ,532a
reached a value of 4.34×10-4 sr−1 due
to pyroCbs 7 and 8 (Table 2) occurred in 2017 (by comparison, Bπ,532a=3.78×10-4 sr−1 in 2016). This
substantiates the assumption that the effect of pyroCbs on aerosol loading
of the UTLS sometimes can be comparable to that of volcanic eruptions (Fromm et al., 2010).

The increasing number and intensity of boreal forest fires in North America
and northeastern Asia due to climate warming over the last decades (Wotton et
al., 2010, 2017; Sofiev et al., 2013; Rémy et al., 2017) lead to an
increasing number of pyroCbs, the plumes of which are able to reach the UTLS
(Fromm et al., 2010; Guan et al., 2010). Boreal wildfires are usually active
during the warm half-year (April to September) and spread in the UTLS for
long distances mainly due to the westerly zonal transport of air masses in
the Northern Hemisphere. Therefore, the plumes of pyroCbs occurring in North
America are frequently detected in the UTLS over Europe, and more rarely
over Siberia, and the Far East by both ground- and space-based lidars.

In this study, we have considered and analyzed aerosol layers in the UTLS
(11–30 km) over Tomsk that could represent smoke plumes from pyroCbs
generated by massive wildfires in North America and northeastern Asia
between 2000 and 2017. Using the HYSPLIT trajectory analysis, we have reliably
assigned 9 such layers to 8 out of more than 100 documented pyroCb
events, the aftereffects of which could potentially be detected at the SLS.
All the eight pyroCb events occurred in North America: one event per year
occurred in 2000, 2002, 2003, 2013, 2015, and 2016, whereas two events occurred in 2017.
Such a small number of observed pyroCbs could be due to unfavorable
weather conditions in Tomsk, or pyroCb plumes could have passed by the SLS.
Unfortunately, we could not unambiguously discern plumes from pyroCbs
occurring in the 2004–2011 period (Table 1) against the background of more
powerful plumes from 12 volcanic eruptions observed during this period (Table 3).

Massive forest fires generating pyroCbs are also known to occur in
northeastern Asia (http://pyrocb.ssec.wisc.edu/, last access: 13 March 2019). However, no plumes in the UTLS over
Tomsk from pyroCbs that occurred in the boreal zone of Siberia and the Far
East (to the east of Tomsk) were detected at the SLS between 2000 and 2017.
We can assume that the time durations for pyroCb plumes to be
detected in the UTLS using ground-based lidars are less than about a month.
In other words, plumes from pyroCbs generated by wildfires to the east of
Tomsk can significantly diffuse before reaching the SLS due to the westerly
zonal transport. This probably explains a comparatively “low” contribution
from pyroCbs to aerosol loading of the UTLS over Tomsk and, therefore, the
negative trends in the annual average Bπ,532a values in
the absence of, and during low, volcanic activity in time periods (a) and (c),
respectively (Fig. 10).

Based on the results of lidar observations at the SLS between 2000 and 2017,
we can conclude the following. During a short-term period (up to 3 weeks) after pyroCb events have occurred in North America, their
aftereffects in the UTLS over Tomsk are comparable to those of volcanic
eruptions with VEI ∼ 3. During periods of volcanic quiescence, smoke
plumes that reached the UTLS over Tomsk from two or more pyroCbs in a single
year can lead to a marked increase in aerosol loading compared to that in
the previous year. For example, the annual average Bπ,532a value
increased by 14.8 % in 2017 compared to that in 2016
due to multiple pyroCbs that occurred in British Columbia (Canada) in
August 2017. The contribution from a single pyroCb or multiple ones
(originating in North America and injecting smoke into the UTLS) to the
annual average Bπ,532a value for the Tomsk region is
comparable to the contribution from both tropical and northern volcanic
eruptions with VEI ≤ 3, but can hardly compete with that from volcanic
eruptions with VEI = 4. The last conclusion is in agreement with the
findings reported by Peterson et al. (2018).

The NOAA's HYSPLIT model used to calculate all air mass backward
trajectories is available at http://ready.arl.noaa.gov/HYSPLIT.php/ (last
access: 13 March 2019). The volcanic eruption data can be found at http://volcano.si.edu/
and the data on pyroCb events that occurred after May 2013 are located at http://pyrocb.ssec.wisc.edu/
(last access: 13 March 2019). The integrated aerosol backscatter coefficient
and scattering ratio data retrieved from the SLS observations are presented
in the Supplement. To open .opj files, the scientific graphing and data analysis
software “Origin” is required (https://www.originlab.com/, last access:
13 March 2019), the trial version of which can be downloaded at
https://www.originlab.com/demodownload.aspx/ (last access: 13 March 2019).
The CALIPSO data used to corroborate the Tomsk observations in the 2013–2017 period
and radiosonde data from meteorological stations located in Kolpashevo, Emeljanovo,
Novosibirsk, and North America are also given in the Supplement.

Aerosol layers detected in the UT–TR with ground-based lidars are identified
as cirrus clouds if the scattering ratio R(H)>10 for a laser
wavelength λ1=532 nm (Tao et al., 2008; Samokhvalov et
al., 2013). However, according to Sassen et al. (1989), the minimum value
of R(H) can be 5.2 in the case of so-called
“subvisual” cirrus clouds, invisible to the naked eye (for a laser wavelength
λ2=694.3 nm),
with a thickness of <1 km. Note, however, that the thickness of other cirrus
cloud types can often be more than 1 km (Goldfarb et al., 2001). To calculate
the minimum R(H) value for λ1=532 nm, one can use the fact that
the aerosol backscatter coefficient βπa(H, λ) is
considered to be independent of the scattered light wavelength if aerosol
particles are much greater than the wavelength (Measures, 1984). Since cirrus
cloud particles (∼25µm, Sassen et al., 1989) are greater than
both considered wavelengths λ1 and λ2, we can assume
βπa(λ1)=βπa(λ2) for
each altitude H. Therefore, using Eq. (1), we can write the following
equality:

(A1)Rλ1-1βπmλ1=Rλ2-1βπmλ2.

Taking into account the dependence βπm(λ)∼λ-4,
for the scattering ratio we have

VVZ and VVG performed the main analysis of all data and wrote
the paper. AVN made measurements at the SLS and processed lidar data.
ESS performed the HYSPLIT trajectory analysis. VVG and ESS retrieved data on
pyroCbs, the plumes of which could potentially be detected in the UTLS over Tomsk.

We thank Michael Fromm (the US Naval Research Laboratory) for the information
on the aerosol cloud coming to Tomsk from several strong pyroCbs generated by
wildfires in British Columbia (Canada) in August 2017.

Massive wildfires sometimes generate pyrocumulonimbus clouds (pyroCbs), inside of which combustion products can ascend to the upper troposphere or even lower stratosphere (UTLS). Smoke plumes from pyroCbs occurred in North America can spread in the UTLS for long distances and be observed in the UTLS over Europe and even over Russia. In this work, we analyzed aerosol layers detected in the UTLS over Tomsk (Russia) that could be smoke plumes from such pyroCbs that occurred in the 2000–2017 period.